This invention relates in general to ceramics and, more particularly, it relates to preparing and firing spheroidal ceramic particles, to preparing useful products such as filter elements of closely controllable porosity therefrom, and to those products.
It has been known for many years to make spheroidal ceramic particles by the process known as spray-drying. Specifically, the ceramic particles are formed into water-based fluid slip, which generally contains additives such as dispersants or deflocculants, wetting agents, and binders. The slip is spun off rotating discs and falls as a fine spray through a column of hot gas. The resulting dry pellets are readily handled and flow into molds much more freely, having a much lower angle of repose (or critical surface angle) than powders not so treated. The application of spray-drying ceramics for use in forming spark plug cores is, for example, disclosed in U.S. Pat. No. 2,251,454 of Jeffery.
More pertinent to the present invention is the even earlier patent of Howe, U.S. Pat. No. 2,007,053, wherein porous granular ceramics are bonded to form filter elements, with uniform and controllable permeability. The patentee states that the raw ceramic powder (silicon carbide or alumina) may be mulled to grind down corners, but permeability is controlled in the first instance by grain size control, and, more importantly, selection of a bonding agent of controlled viscosity at the bonding temperature. The bonding agent is generally low-melting compared to the refractory material, e.g. ball clay, slip clay and/or feldspar. The essential concept is to control how the bonding agent flows over and coats the refractory particles during firing.
Other methods of making spheroidal ceramic pellets for particular applications are known but are less pertinent to the present invention. Thus, particles for use in molecular sieves are extruded into narrow strands and dried under conditions controlled to form individual uniform particles (U.S. Pat. Nos. 2,872,719 of Brassfield et al and 3,287,472 of Wolf et al). Lightweight, foamed ceramic particles are produced from slips by a variety of methods (U.S. Pat. Nos. 2,399,225 of Heany, 3,232,772 of Hilton et al, and 4,057,605 of Chauvin). Particles for use in catalysis or pebble heater applications fall in the same general category, but are so much larger than those used in the present invention that they are not deemed pertinent.